On Ubuntu 14.04.2, with both the stock kernel and 3.19.0-22, nouveau fills kern.log with messages like these:

====================8< CUT HERE 8<=================================================================
$ tail -n 3000 kern.log.1 | head -n 50
Jul 10 18:02:35 dioo-XPS kernel: [114893.357149] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.406261] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.406269] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.420442] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.420447] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.479210] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.479217] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.568349] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.568356] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.622806] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.622813] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.637217] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.637221] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.695836] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.695843] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.784309] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.784316] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.838180] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.838187] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.853754] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.853759] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.913521] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.913528] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.977242] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.977249] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.068780] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.068788] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.122599] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.122606] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.137025] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.137030] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.195675] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.195682] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.284069] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.284078] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.339830] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.339837] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.353411] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.353416] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.412188] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.412204] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.501906] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.501913] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.558826] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.558831] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.570001] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.570006] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.628831] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.628839] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.717193] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
====================8< CUT HERE 8<==================================================================

On Kernel 4.2.0-rc2 (compiled based on the stock kernel's config), the computer just locks up:

I xscreensaver-command -lock, then logged back in (no issue there), then I xscreensaver-command -lock again and the computer just locked up. Caps lock light was unresponsive on the keyboard, Ctrl+Alt+Delete didn't work, RESIUB magic keys either.

After hard rebooting the computer, here is what what in kern.log and syslog:

Jul 16 17:19:45 dioo-XPS rtkit-daemon[2678]: Supervising 4 threads of 1 processes of 1 users.
Jul 16 17:27:15 dioo-XPS kernel: [ 619.825758] nouveau E[ PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
Jul 16 17:27:15 dioo-XPS kernel: [ 619.825764] nouveau E[ PFIFO][0000:01:00.0] PGRAPH engine fault on channel 2, recovering...
Jul 16 17:28:35 dioo-XPS dbus[894]: [system] Activating service name='org.freedesktop.systemd1' (using servicehelper)

The downstream bug is https://bugs.launchpad.net/ubuntu/+source/linux-lts-vivid/+bug/1474538